1. Field of the Invention
Embodiments of the present invention generally relate to method and apparatus for semiconductor substrate processing. More specifically, embodiments of the present invention relate to methods and apparatus for physical vapor deposition.
2. Description of the Related Art
Physical vapor deposition is performed in chambers at low pressure and high temperature. To provide low pressure environments such as 5*10−8 Torr or less for ultra high vacuum (UHV), pumping systems are selected with optimum efficiency. A cryogenic pump is often selected to provide high gas pumping speeds. A first stage of the pump is maintained at 60-100K and the second stage is maintained at a lower temperature, around 13K. When the PVD chamber is cleaned or other maintenance is performed, water is adsorbed on the internal chamber surfaces. As the chamber gases are pumped out of the chamber, the rate of pressure drop decreases as the evaporation and removal of water adsorbed on the chamber surfaces becomes the determining rate step in the evacuation of the chamber. To achieve UHV within an acceptable time period, the chamber is heated with infrared (IR) lamps. The IR energy heats the internal surfaces and energizes the water molecules to break the weak water-water bonds and desorb. This bake out process helps obtain a chamber pressure of 5*10−8 Torr within hours.
This relatively fast chamber bake out requires an IR energy input in the range of tens of mW/cm2. The cryogenic pump is not as effective (its surface temperature rises) if exposed to energy greater than about 50 W. Water that has adsorbed on the pump surface may even reach vapor pressure and release back into the chamber. Protection of the cryogenic pump assembly is needed to protect the cryogenic pump, prevent trapped gas volumes, provide limited down time for cleaning and maintenance, and require low cost manufacturing and installation or retrofit.